Guest Post by Willis Eschenbach
Anthony has discussed a paywalled study in the new reality-based Nature Magazine production, Nature Climate Change magazine. Unlike Anthony, they approved my application for a free subscription … go figure. The study is called “Nonlinear heat effects on African maize (corn) as evidenced by historical yield trials”, Lobell et al. (hereinafter L2011). The study looked at the effect of heat on corn production. Here’s their Figure 1:
Their conclusion? When it gets above a certain temperature, maize growth quickly slows, and it’s worse when it’s dry. Of course with the obligatory links to global warming and the danger of large drops in corn production. Shocking news, I know. They provided a citation to other scientists saying the same thing, in case you doubted it — too much heat is bad for plants. I bet the farmers of the world were as amazed as I was.
Or as they put it in their abstract:
Each degree day spent above 30° C [86°F] reduced the final yield by 1% under optimal rain-fed conditions, and by 1.7% under drought conditions. These results are consistent with studies of temperate maize germplasm in other regions, and indicate the key role of moisture in maize’s ability to cope with heat.
Now, we need to be careful here. They are not talking about the number of days where the temperature goes above 30°C. They are discussing “degree days”. That is the sum of the average daily temperature (C) less 30 degrees, for all the days where the average temperature [defined as (daily max + min)/2] is above 30°C. The figure is written as “GDD30+”, for “growing season degree days over 30°C”. They figure the growing season as 150 days, which agrees with the Texas figures given below.
Are their numbers accurate? Is there a drop in yield of 1% for every degree day as they claim? I don’t know. Haven’t done my homework yet, just dug up the paper, gimme a minute. Where do they grow corn? Iowa? Let me look it up. OK, I find:
Fascinating. I love doing this, I get to learn so much. Well, at first glance I’d say the following:
1. The major corn-growing areas are from about 37°N to 47°N. So clearly, corn prefers temperate weather.
2. Corn is only a minor crop in many regions within that general preferred temperature band. So obviously, there’s other factors. The usual suspect would be water, second would be soil.
3. Corn is grown in the California Central Valley, one county in Arizona (irrigated, no doubt), a number of counties in southern Texas (mostly irrigated), and one county in Florida. I looked at the temperature record for Hidalgo County, the left one of the counties at the south tip of Texas in Figure 2. I looked at the daily temperature record for Edinburgh, in the middle of the county.
Here’s the curious thing. During the corn-growing season of 1999, the total number of “degree day[s] spent above 30° C” (GDD30+) in the Texas corn-growing area was 136 … so if yield dropped by 1% for each degree-day over 30°C, we’re down
below zero to a quarter of the original yield. Hmmm. Figure 3 shows the degree day analysis, from the excellent online calculator from Wolfram Alpha here:
Figure 3. Degree days over 30°C for 150-day 1999 corn-growing season, Edinburgh, Texas.
I got to thinking about what was happening. How could they be growing corn in that kind of heat, with a GDD30+ over a hundred and thirty? I thought about it a while, and looked around on the web a bit. Figure 4 shows part of the answer:
Figure 4. Corn planting and harvesting dates in Texas. The “Panhandle” is the most northerly square section of the state (see Figure 2). SOURCE.
I’m sure you see the pattern. In the south, like Hidalgo County above, they plant and harvest early. Their crop is three-quarters harvested before the rest of the state has even begun.
As for the other part of the answer, I don’t know. I don’t know why even with their early growing season (March 1 – August 1) the Texas farmers are still able to grow corn in that heat. The L2011 study says that’s impossible, but perhaps the Texas guys and gals didn’t get the memo, they’re a cactus-tough bunch down there, hard to get hold of. Thinking on it, though, it’s more likely they got the memo and shot it full of holes for target practice. In any case, during their growing season, the Texas farmers have no less than a hundred and thirty-six degree days over 30°C, which according to the L2011 results should reduce yield by
136% 75% … which means that either I or Wolfram or the climate scientists did something wrong. I’m open to any suggestions, I’ve been wrong before.
Now, if there were to be a general warming, say a degree on average over some long time, what do you think will happen to the planting and harvesting dates in Figure 4? Do you think those farmers would keep planting at the same time of year, year after year, in the face of increasing hot days summer and decreasing yield? Do we really face a 1% drop in yield for every degree day over 30°C?
Naw … in answer to the question in the title of this post, farmers are smarter than the L2011 climate scientists. If temperatures change, the farmers change their planting times … what do you do?
My best to everyone.